1. Field of the Invention
The invention relates to a method of producing aspherical optical surfaces of optical elements, in particular for use in microlithography for producing semiconductor elements.
2. Description of the Related Art
U.S. Pat. No. 6,373,552 discloses a method of producing an aspherical surface profile on a plane-parallel plate to which a material has been applied. A thin layer is applied to the plate, the plate then being turned over and placed with the layer onto a vacuum table and sucked into place. The surface remaining free is polished flat. This produces a new surface, the plate being of a constant thickness. After releasing the plate, it is once again turned over, sucked into place again by the vacuum table and machined flat, the original aspherical profile being removed and consequently a new surface once again being created. After removal of the plate from the vacuum table, a plate of a constant thickness with the desired aspherical surface on both sides is obtained.
However, a factor contributing to the inaccuracy of this method is that short-wave and fine structures that have been produced during the machining operation on the vacuum table are transferred to the plane-parallel plate to be machined.
Furthermore, U.S. Pat. No. 3,837,125 discloses a holding device for receiving a lens blank in a machine for grinding aspherical lens surfaces. In the case of the holding device, the lens to be machined is sucked into place onto a lens holder, which has a base surface which is formed inversely in relation to the desired aspherical lens surface. The surface of the lens to be machined that is not resting on the base surface of the lens holder is ground flat. When the machined lens body is released, the lens surface assumes the desired aspherical form. Consequently, membranes that are subjected to force by actuators are used during the polishing operation.
However, this arrangement has the disadvantage that the actuators result in undesired removal of material at the edge region. The machining of the edge region of an optical surface is extremely difficult with such a membrane.
Furthermore, the production of aspherical lenses or mirror surfaces by means of a molding technique is generally known. It is critical, however, that an epoxy resin which is used for replication in a molding technique of this type remains a component part of an optical surface. The method can be used only conditionally in the production of optical surfaces with large diameters, for example in the range of 10-30 cm, since a “curling” of the epoxy resin occurs, and consequently the surfaces produced with this method are no longer usable for each precision.